Perception of odor is a physical mechanism by which information is processed in the brain. Humans are able to detect an unlimited number of dissimilar odors and in dilutions of up to one part in 109. It is believed that the brain is able to evaluate and identify tens or possibly hundreds of thousands of different odors. The brain is able to recognize these odors and associate them with likes, dislikes, events and experiences. According to the chemical theory of olfaction, molecules of odors, or odorants, are conveyed to the olfactory epithelium by convection, diffusion or both and directly or indirectly induce changes in the olfactory receptors.
It is believed that the human nose detects smells when odorants strike a region on the olfactory neurons, the cells that contain the odorant receptors. Axons extend from these cells to the olfactory bulb, which is the brain region that processes olfactory information. Recently, it has been found that individual odorants activate multiple receptors. Moreover, individual receptors respond to multiple odorant (see, B. Malnic et al., Cell, (1999) Vol 96, 713-723). As described in the foregoing article, the researchers concluded that the combination of receptors that the activated by an odorant determines the smell that humans perceive. It is believed that odorants activate a combination of olfactory neurons giving rise to a combinatorial code, which encodes odorant identities.
Moreover, it is known that a slight change in an odorant's structure can have a dramatic affect on how the odorant is perceived. Octanol has a sweet, orange, rosy, fresh and waxy smell, whereas octanoic acid has a rancid, sour, repulsive and sweaty smell. Although these odorants differ only by the oxidation state of one carbon atom, they have markedly different perceived smells. In this instance, the foregoing study concluded that the odorants triggered overlapping, yet distinct odorant receptors.
The human nose measures odors by their intensity. The threshold value of one odor to another can vary greatly. The detection threshold is the minimum intensity necessary for detection without necessarily identifying the odor. In human olfaction, high odor detection thresholds are observed for odorants that are gases under standard pressure and temperature conditions. Odorants with low vapor pressures generally have low odor detection thresholds (see, Devos, M. et al., Standardized Human Olfactory Thresholds, (Oxford University Press, New York), pp. 165 (1990)).
An electronic nose is an instrument used to detect vapors or chemical analytes in gases, solutions, and solids. In general, an electronic nose is a system having an array of sensors that are used in conjunction with pattern-recognition algorithms. Using the combination of an array of sensors, which produce a fingerprint of the vapor or gas, the recognition algorithms can identify and/or quantify the analyte(s) of interest. The electronic nose is thus capable of recognizing unknown odorants.
During use, an electronic nose is presented with a substance, such as an odor or vapor, and the sensor converts the input of the substance into a response, such as an electrical response. The response is then compared to known responses that have been stored previously. By comparing the unique chemical signature of an unknown substance to “signatures” of known substances, the unknown analyte can be determined. A variety of sensors can be used in electronic noses that respond to various classes of gases and odors.
In an effort to construct better electronic noses, attempts have been made to understand odorant detection thresholds that are displayed by the human olfactory sense. Moreover, in an attempt to correlate trends in odor intensity with specific microscopic and macroscopic properties of various odorants, structure-activity relationships have been formulated. For example, researchers have proposed that trends in detection thresholds arise from the presence of steric and other functional groups in olfactory receptors (see, Ohloff, G., Scent and Fragrances, the Fashion of Odors and Their Chemical Perspectives, (Springer-Verlag, New York), pp. 238 (1994); Amoore, J. E., Molecular Basis of Odour, (Charles C. Thomas, Springfield, Ill., pp. 200 (1970)). Such receptors can then respond to features such as molecular length and polarity (see, Amoore, J. E., Molecular Basis of Odour, (Charles C. Thomas, Springfield, Ill.), pp. 200 (1970); Dravnieks, A. in Flavor Quality: Objective Measurement, (American Chemical Society, Washington), pp. 11-28 (1977); Edwards, P. A. et al., Chemical Senses 14, 281-291 (1989); Edwards, et al., Chemical Senses 16, 447-465 (1991)). Other researchers have empirically correlated odor detection thresholds with macroscopic properties of the odorant such as the boiling point of the odorant molecules (see, Abraham, M. H. in Indoor Air and Human Health, (CRC Press, New York), pp. 67-91 (1996); Greenberg, M. J. in Odor Quality and Chemical Structure, (American Chemical Society, Washington), pp. 177-194 (198 1); Laffort, P. et al., N.Y. Acad. Sci. 237, 192 (1974)). Further, other researchers have noted the correlation between odor thresholds and the vapor pressure of the odorant (Moulton, D. G. et al., Quart. J. Exp. Psychol. 12, 99-109 (1960); Cometto-Muñiz, J. E. et al., Indoor Air 4, 140-145 (1994); Cometto-Muñiz, J. E. et al., Physiol. Behav. 48, 719-725 (1990); Mullens, L. J., Ann. New York Acad. Sci. 62, 247-276 (1955); Ottonson, D. Acta Physio. Scand. 43, 167-181 (1958)).
In view of the foregoing, what is needed in the art is a method that can match the response intensity of a sensor array to an odorant with the detection threshold of a human nose responding to the same odorant. The present invention fulfills this and other needs.